WO2016/037696A1 discloses as a conventional compression ratio variable mechanism able to change a mechanical compression ratio of an internal combustion engine a mechanism where a small end shaft receiving hole formed at a small end of a connecting rod has inserted into it, to be able to rotate, an eccentric member supporting a piston pin so that an axial center of the piston pin is positioned at a position offset from a center axis of the small end shaft receiving hole by exactly a predetermined amount. According to the constitution of the conventional variable compression ratio mechanism, by making the eccentric member turn within a predetermined range of rotational angle in both directions, it is possible to make the axial center of the piston pin move up and down (cylinder axial direction) about the center axis of the small end shaft receiving hole. Due to this, it is possible to make the length from the axial center of the piston pin to the axial center of the crank pin (below, referred to as the “connecting rod length”) change. As a result, for example, if lengthening the connecting rod length, the volume of a combustion chamber when a piston is positioned at compression top dead center becomes smaller, so it is possible to raise the mechanical compression ratio of the internal combustion engine. On the other hand, conversely, if making the connecting rod length shorter, the volume of the combustion chamber when the piston is positioned at compression top dead center becomes larger, so the mechanical compression ratio of the internal combustion engine can be lowered.